The present invention is directed to projection prisms, and more particularly to color projection prisms for dispersing and recombining light.
Color dispersement and recombination are important aspects of color projection systems. In these systems, white light is created from, for example, an arc lamp. A prism or other such device is used to disperse the white light into three components: a red beam, a green beam and a blue beam. These beams may be directed to reflective liquid crystal cells that interfere with and selectively reflect each beam. In other words, the interference imparts an image to each color beam. More particularly, the reflective liquid crystal cells are often active matrix cells with a switching element for each pixel allowing each pixel to be individually addressed. The colors are recombined, and when projected on a screen form a full color image.
U.S. Pat. No. 4,943,154, issued Jul. 24, 1990, discloses a prior art projection system, as shown in FIG. 1. This system comprises a light source 50; a light transmitter 59; light valves 61, 62 and 63; a light combiner 64; a projection lens 65; a drive circuit 67; an input side convergent lens 68; an input side plane mirror 69; a central convergent lens 70; an output side plane mirror 71; and an output side convergent lens 72. The light source 50 comprises a lamp 51, a condenser lens 52, a concave mirror 53, and a heat absorbing filter 54. The lamp 51 radiates a light containing three primary colors of red, green and blue. Rays of the radiant light from the lamp 51 are arranged in approximately parallel rays by the condenser lens 52 and the concave mirror 53. More specifically, the rays of light from the center of a luminous element 55 in the lamp 51 are transmitted in parallel to the optic axis 57 by the condenser lens 52. From the light rays passed through the condenser lens 52, infrared rays are eliminated by the heat absorbing filter 54. The light rays from the light source 50 are separated into three primary colors of red, green and blue. Red light passes through the light transmitter 59 and enters the red light valve 61. Green light is similarly transmitted to the green light valve 62., and blue light is transmitted to the blue light valve 63.
A light separator 90 is used to disperse the light into colored beams of red, green and blue. Separator 90 is shown having two plates 92 and 94. The transmissive and reflective properties of these plates are shown in the graphs of FIGS. 2(a) and 2(b). FIG. 2(a) is a graph of transmission (in percent) as a function of wavelength (in nanometers (nm)) for plates 92 and 94, corresponding to curves A and B, respectively. FIG. 2(b) is a graph of reflection (in percent) as a function of wavelength (in nanometers) for the same plates. In particular, FIG. 2(a) shows that plate 92, corresponding to curve A, has a low transmissivity for blue light but a high transmissivity for green and red. Plate 94, corresponding to curve B, has a low transmissivity for blue and green light but a high transmissivity for red. Put another way, and as shown in FIG. 2(b), plate 92 reflects the blue light but transmits green and red. Of the green and red light that remains, plate 94 reflects the green but transmits the red. Thus, the two plates may be used to disperse the light before the light enters light valves 61, 62 and 63.
The light valves 61, 62 and 63 are liquid crystal panels each having matrix electrodes. The drive circuit 67 produces electric signals R, G and B according to a video signal Y to control the transmittance of pixels in respective light valves 61, 62 and 63. The modulated light outputs from the light valves 61, 62 and 63 are then combined into a composite flux of light substantially to reproduce a color picture at the position of the light valve 62. An enlarged image of the color picture is finally projected by the projection lens 65 onto a screen 66.
Light combiner 64 may also be used to disperse the light into the different colors (this system is not shown). In this case, the light disperser/combiner often includes dichroic surfaces such as coated plates of glass or an internally-coated cemented prism structure. Coatings used as dichroic surfaces are typically thin films.
A disadvantage of coated plates is that such plates can introduce astigmatism in the optical path. Inserting additional non-coated plates may compensate for this defect at the expense of simplicity.
Prism structures have many advantages. They are typically rugged, easy to mount, and useful for beam superposition applications. They deform less when subjected to stress than coated plates. Coatings used in prisms are resistant to degradation because they are usually sealed within the body of the cube.
Prism structures may also have some disadvantages. One may be a polarization dependence of the reflected light arising from the coatings for the interior surfaces. These coatings may be multilayered and reflective. As the angle of the plane of the coating to the direction of light moves away from 90.degree., spectral differences between the polarization components increase. In other words, the reflected light becomes at least partially polarized for some range of wavelengths, referred to as the transition region.
This effect may be seen, e.g., in FIG. 3(a), which shows the corresponding curves for transmittance for a short-wave-pass beamsplitter or prism at 45.degree.. In FIG. 3(a), the transmission (in percent) is plotted versus wavelength (in nm). The action of the beamsplitter is seen to be dependent on the polarization of the light. S-polarized light, i.e., light which has its electric field perpendicular to the plane of incidence, has a transmittance curve which is shifted relative to that for p-polarized light, the latter being light which has its electric field parallel to the plane of incidence. The amount of shifting is seen to be roughly 50 nm towards lower wavelengths. Thus, the transition region, in which polarization components may be subject to spectral differences, is about 50 nm wide.
Some beamsplitters have been constructed which reduce the spectral difference between the polarization components. FIG. 3(b) shows plots of wavelength (in nm) versus reflection (in percent) for light of random polarization (*), s-polarization (+), and p-polarization (.times.) for a blue mirror made by Doctor Optic, GmbH of Vienna, Austria. It is seen that the s-polarization component is shifted from the p-polarization component by about 5 nm. Thus, the transition region, in which polarization components may be subject to spectral differences, is much smaller and is about 5 nm wide.
The partially polarized light is usually incident on the cell which imparts an image to the light. The cell, which may use a scattering liquid crystal material, may have at least a partial depolarizing effect. This may be undesirable. For example, if the incident beam is highly polarized, and the cell depolarizes that same state of polarization, then the intensity of the beam passing back through the prism may be attenuated. In extreme cases, the intensity may drop to about half the incident intensity because the beam is not properly reflected.
This type of attenuation is generally caused by each of the three cells. The overall effect is to reduce the transmission of the prism system. For example, a typical ratio of the amount of light exiting the prism system to that entering is about fifty percent for dichroic surfaces that are oriented 45.degree. to the direction of the light. For cells that use polarization of the light for their optical action, this polarizing effect can be used to increase their contrast ratio. However, this may severely attenuate the light intensity, perhaps by about sixty percent.
Some solutions have been proposed for the above problems. For example, U.S. Pat. No. 4,969,730, issued Nov. 13, 1990, discloses a prism system with two air gaps. While these air gaps may reduce attenuation, they may produce undesirable "ghost" images.
Accordingly, an object of the invention is to provide an optical prism in which the ratio of the amount of light exiting the prism system to the amount entering is maximized.
Another object is to provide a prism system for color dispersement and recombination in which dispersement occurs while introducing a minimum of polarization in the dispersed light.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.